Commercially available hydrofluoric acid detectors, which can be based on infrared (IR) light absorption using Tunable Diode Lasers (TDL), are used in alkylation units in petrochemical refineries to monitor background HF concentrations, detect accidental releases for Safety, Health and Environmental (SHE) applications and to initiate mitigation actions. These sensors target HF monomers as the light absorbing species. The TDL light source generates light that is absorbed by HF in a very narrow wavelength range, making the detectors very specific to HF.
HF has three pairs of localized sp3 electrons that do not participate in covalent bonding. These non-bonding electrons make HF a very strong hydrogen bonding molecule. This is typical of hydrides formed by the second row elements in the periodic table (e.g., NH3, H2O and HF). The strong tendency to form hydrogen bonds leads to the formation of gaseous polymers of HF (e.g., (HF)2, (HF)4 and (HF)6). In addition, this same characteristic leads to the formation of hydrates (i.e., water molecules hydrogen bonded to HF). The formation of polymers and hydrates broadens and distorts the HF absorption spectrum.
Nevertheless, commercially available TDL detectors target the monomer for detection. The abundance of the HF monomer in ambient air is dependent on the ambient temperature and humidity, since some of the monomers that would otherwise be present form hydrates at higher humidities. As a result, the actual HF concentration in the airspace can be underreported. Effects of underreporting are amplified in newer point detectors based on TDL technologies that allow for detection at low concentration levels of interest (HF TLV=3 ppmv), and reduced interference from other chemical compounds. Even in earlier open path TDL models, that provide an average concentration over the monitored line of sight path, the average measured value would still be underreported. Thus, the open path sensors could also benefit by the application of the same ambient humidity and temperature correction.
Thus, there remains a need for HF detectors that can accurately output the true amount of HF in an air space, including HF that is contributed in hydrate form.